Loud speaker cabinet

ABSTRACT

A loudspeaker system is provided comprising a vibration-free cabinet of minimum interior size having a relatively thick front wall including two spaced parallel sound exit apertures. A loudspeaker is attached to the rear of the front wall with its cone communicating directly with one of the exit apertures, and a sound-delay chamber of constant cross-section throughout its length is interposed between the second exit aperture and the interior of the cabinet, leading to the rear of the speaker cone. Sound waves from the two resulting sound paths reinforce each other in a slightly out-of-phase relationship which enhances the quality and depth of the resulting overall sound and utilizes the full power of the speaker. Also, microphone-speaker feedback is substantially eliminated at normal power and at locations beyond approximately one foot from the front of the cabinet.

United States Patent 1 Monroe 1 Jan. 23, 1973 1 LOUD SPEAKER CABINET vABSTRACT [76] lnventor: Donald V. Monroe, 1629 N. Del A loudspeakersystem is provided comprising a vibra Mar, Rosemead, Calif. tion-freecabinet of minimum interior size having a relatively thick front wallincluding two spaced paral- [22] Flled' March 1972 lel sound exitapertures. A loudspeaker is attached to [21] Appl. No.: 235,714 the rearof the front wall with its cone communicating directly with one of theexit apertures, and a sounddelay chamber of constant cross-sectionthroughout its Y' length is interposed between the second exit aperture[5 1 'f 18 r x and the interior of the cabinet, leading to the rear of[58] Fle d of Search 1/3 B the Speaker cone Sound waves from the tworesulting sound paths reinforce each other in a slightly out-of- [56]Reerences cued phase relationship which enhances the quality and UNITEDSTATES PATENTS depth of the resulting overall sound and utilizes thefull power of the speaker. Also, microphone-speaker 1,869,178 7/1932Thuras ..181/3l B feedback is substantially eliminated at normal power2,315,896 4/1943 Dumas and at locations beyond approximately one footfrom 2,900,040 8/1959 Novak the from f the cabinet 2,971,598 2/1961Sieler 3,037,081 5/1962 Carlsson ..18l/31 B Primary ExaminerStephen .l.Tomsky Attorney Fred N. Schwend \l lm lllllflfllfllll- 7 Claims, 3Drawing Figures LOUD SPEAKER CABINET BACKGROUND OF THE INVENTION 1.Field of the Invention This invention relates to loudspeakers and hasparticular reference to loudspeaker systems capable of being universallyused for music, paging, and public address applications.

2. Description of the Prior Art One of the most perplexing problemsencountered in microphone loud speaker combinations, as found, forexample, in paging and public address systems, is the development ofundesired feedback noise which occurs when the microphone is placed inthe neighborhood of the speaker to which it is electrically connected.Although attempts have been made to solve this problem, such solutionshave generally resulted in expensive construction, impaired soundquality and/or reduced power output of the speaker. Also, heretofore, inorder to provide clear and faithful or authentic reproduction of soundthroughout the normal audio range and at relatively full power, variousexpedients have been used. For example, combinations of different sizeloudspeakers are employed in many cases, in which larger speakers areused for transmitting lower frequency sounds and smaller speakers areused for transmitting higher frequency sounds. Such combinations havecome into almost universal use for several reasons. One is that two orthree of such speakers could be crowded into a smaller cabinet of priordesign than could a single full-range speaker. Another is that maximumpower rating increases with each added speaker, making it possible forless expensive speakers to be used.

SUMMARY OF THE INVENTION tively thick front wall, i.e., on the order oflAinches thick, in which two spaced parallel sound exit apertures, onthe order of 4 inches indiameter each are formed. Because of thethickness of the front wall, the exit apertures assume the form of soundconduits, each having a length of approximately one-third of itsdiameter.

A loudspeaker is attached behind the front wall in line with one of thesound exit apertures and a sound delay chamber having a constantcross-section throughout its length is formed intermediate the secondsound exit aperture and the interior of the cabinet leading to the rearof the speaker cone. Such chamber has a cross-sectional area ofsubstantially 6%square inches and an effective length'of substantially%inches. It has been found that this size chamber is effective toslightly delay the passage of substantially all sound waves within thenormal audio spectrum. The resulting two sound paths appear to reinforceeach other to utilize substantially the full power of the speaker. Also,due to the resulting slight out-of-phase relationship of the two soundsthus emanated, a slight echo effect results which adds to the depth andrichness of the overall sound. This, combined with the particular sizeand configuration of the cabinet results in a clear and authentic orfaithful reproduction of music or the like throughout substantially theentire audio range. Further, and of considerable importance, is theabsence of any microphone speaker feedback tendencies beyond a distanceof approximately 1 foot from the front wall of the speaker. Apparently,this is due to the fact that the two sounds, when combined at a shortdistance from the front of the speaker, result in a total sound which issomewhat out of phase with the feedback characteristics of themicrophone speaker combination.

It has been found, in using a five inch speaker with sound exitapertures on the order of 4 inches in diameter, that it is essential tomaintain the length of the opening of such openings (thickness of thefront wall) at least 1%inches in order to restrict the point at whichfeedback occurs to within 1 foot of such exit apertures, which is wellwithin desirable limits and well within the limits of other speakercabinets of which applicant is aware.

Accordingly, a principal object of the present invention is to reducemicrophone loudspeaker feedback tendencies in loudspeaker systems.

Another object is to provide a loudspeaker system capable of faithfullyreproducing all sounds within the normal audio range and at all normalvolume levels.

Another object is to reduce the size of a loudspeaker cabinet withoutimpairing its sound propagating qualities.

Another object is to provide a loudspeaker system having pleasingresonance qualities throughout the normal audio range and at all normalvolume levels.

Another object is to provide a simple and economically manufacturedloudspeaker cabinet capable of faithfully reproducing all sounds withinthe normal audio range and at all normal volume levels.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view with oneside section cut away, illustrating a preferred form of the presentinvention.

FIG. 2 is a sectional elevation view taken along line 2-2 of FIG. 3.

FIG. 3 is a sectional plan view taken along line 3-3 of FIG. 2. I

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring in detail to thedrawings, the cabinet, generally indicated at 11, comprises top andbottom walls l2, 13, front and back walls 14 and I5 and side walls 16,17. The various walls are formed of a dense material commerciallyavailable under the names high density particle boar or high densityflake board". Such walls, except the front wall 14, are at least 3/4inches thick and are chosen to prevent absorption of sound and tosubstantially prevent any undesirable or uncontrolled vibrations frombeing set up in the walls which would otherwise tend to distort thesound emanating from the cabinet. Thin sheets of hard wood veneer (notshown) are bonded to opposite surfaces of the walls to prevent warpageand to reflect rather than absorb sound waves impinging thereon.

Although the side, top and front walls are rigid enough to prevent anyunwanted vibrations from being set up therein, they resonate in the samemanner as a violin or guitar sound box to enhance all sounds generatedby the back of the speaker cone.

The front wall 14 is formed of two panels 18 and 20 having a combinedthickness of l Krinches. Two parallel cylindrical apertures 21 and 22 of4 inches diameter each, are formed through the two panels 18 and 20. Adecorative cloth fabric 23 is interposed between the two panels toextend across the apertures 21 and 22. Such panels are integrallysecured together by a suitable adhesive which also supports the fabric23 in a relatively taut condition across the apertures 21 and 22.

The top, side and bottom walls are integrally secured together with asuitable adhesive and are recessed as indicated at 24 to receive thefront wall 14 which is integrally secured within such recesses by asuitable adhesive to form an acoustically insulated cabinet.

The top, bottom and side walls are also recessed at 25 toreceive therear wall which is removably secured in place by suitable screws 19 orthe like to permit access to the interior of the cabinet. Sealingmaterial may be interposed between the rear wall 15 and the recess 25 toinsure adequate sound insulation at such point. A layer of sponge rubber26 approximately kinch thick and having a substantially smooth soundreflecting inner surface is suitably bonded to the inner surface of therear wall 15 to dampen any unwanted vibrations which might possiblyotherwise be set up in the rear wall. It will be noted, however, thatthe layer 26 will reflect substantially all sound waves generated by thespeaker cone.

A 5inch heavy duty loudspeaker 27 of conventional construction andhaving the usual air moving cone (not shown) is secured by bolts 28 tothe rear panel 18 directly in line with the upper aperture 21. Forexample, such a speaker may be commercially obtained under the tradenameTrusonic Lifetime Speaker from TrusonicAudio of Huntington, lnd. Suchspeaker has a cone capable of a maximum excursion ofapproximately inchand is rated at 40 watts, peak power. A sound delay conduit or chamber30 of constant cross-section throughout its length is provided, openinginto the lower sound exit opening 22, and is formed by parallel walls 31and 32 which extend across the interior of the cabinet 11 and areintegrally secured to spacer strips 33 and 34 attached to the side walls16 and 17. Such chamber 30 has a cross-sectional opening of llfiinchesdeep and 5%inches wide. The chamber has an effective length of 5%inches.

The rear chamber wall 32 is spaced above the bottom wall 13 to form asound entrance 35 and is attached at its upper end to a horizontal wallsection 36 secured to the rear panel 18 of the front wall. The frontchamber wall 31 is spaced inwardly from the panel 18 and terminatesbelow the wall section 36 to form a sound outlet 37 leading to the exitaperture22. All such chamber walls have smooth, non-sound absorbingsurfaces. 1

The cabinet 11 has the following internal dimenstons:

Height (A) equals l2%inches.

Width (8) equals 6%inches.

Depth (C) equals 5%inches.

'cone and from the rear thereof without noticeable attenuation at anypoint in such range. Apparently, although the sound waves transmitted bydelay chamber 30 are slightly delayed or out-of-phase, they reinforcethe sound waves emanating through the aperture 21 at some distance infront of the cabinet to fully utilize the air moving capabilities of thespeaker 27. This out-of-phase relationship creates a slight echo effectto produce a pleasing, clear and natural sound which appears to haveconsiderable depth and in which no distortion is detectable by the humanear. Of considerable importance is the fact that microphone speakerfeedback is eliminated at distances greater than approximately one footfrom the front of the speaker apertures 21 and 22. That is, if amicrophone which is connected incircuit with the speaker 27 through anamplifier is held directly in front of the aperture 21 and substantiallywithin 1 foot of the front wall 14, the usual high pitched feedback willbe heard. If the microphone is held directly in front of the aperture 22and substantially within 1 foot fro the front panel, a much lowerpitched feedback will be heard. However, if the microphone is held inany position relative to the cabinet, but beyond a boundary ofapproximately 1 foot from the front wall 14, no feedback is discernible.

It should be noted that the size and length of the sound exit apertures21 and 22, the size and location of the sound delay chamber 30 and thesize of the interior of the cabinet are found substantially critical tothe above noted elimination of feedback tendencies outside of said 1foot boundary.

As an example of the power handling capabilities of the speakebcabinetit has been found in actual tests that two such loudspeaker-cabinetsprovide adequate sound for transmission of music and voice communicationto an assemblage of 200 people. Also, two such speaker-cabinets providevadequate sound for paging persons within an area of 12,000 square feet.j

I claim:

1. A loudspeaker system comprising'a cabinet including front, rear,side, and end walls, each of said walls being of a thickness sufficientto substantially prevent vibration thereof, 0

a pair of spaced sound exit apertures in said front wall,

a loudspeaker in said cabinet and attached to thefront wall incommunication with one of said apertures,

a sound delay chamber intermediate the other of said apertures and theinterior of said cabinet,

said chamber having a constant cross-sectional area throughout thelength thereof effective to delay substantially all sounds passingtherethrough, and

said front wall having a thickness such that each of said apertures hasa length approximately equal to one third the average distance acrosssaid aperture.

2. A loudspeaker system according to claim 1 wherein said apertures arecircular and are each on the order of 4 inches in diameter and saidfront wall is on the order of llfiinches thick.

3. A loudspeaker system according to claim 1 wherein said chamberextends parallel to said first wall and has an outlet openingcommunicating with said other of said apertures and an inlet openingcommunicating with said interior of said cabinet.

4. A loudspeaker system according to claim 3 1 i l l

1. A loudspeaker system comprising a cabinet including front, rear,side, and end walls, each of said walls being of a thickness sufficientto substantially prevent vibration thereof, a pair of spaced sound exitapertures in said front wall, a loudspeaker in said cabinet and attachedto the front wall in communication with one of said apertures, a sounddelay chamber intermediate the other of said apertures and the interiorof said cabinet, said chamber having a constant cross-sectional areathroughout the length thereof effective to delay substantially allsounds passing therethrough, and said front wall having a thickness suchthat each of said apertures has a length approximately equal to onethird the average distance across said aperture.
 2. A loudspeaker systemaccording to claim 1 wherein said apertures are circular and are each onthe order of 4 inches in diameter and said front wall is on the order of1 1/4 inches thick.
 3. A loudspeaker system according to claim 1 whereinsaid chamber extends parallel to said first wall and has an outletopening communicating with said other of said apertures and an inletopening communicating with said interior of said cabinet.
 4. Aloudspeaker system according to claim 3 wherein said delay chamber has across-sectional area on the order of 6 1/4 square inches and a length onthe order of 5 1/4 inches.
 5. A loudspeaker system according to claim 3wherein the interior of said cabinet is on the order of 12 1/4 incheshigh, 6 1/4 inches wide and 6 inches deep.
 6. A loudspeaker systemaccording to claim 5 wherein said inlet opening is adjacent the said endwall remote from said first mentioned aperture.
 7. A loudspeaker systemaccording to claim 6 wherein said chamber extends across said otheraperture.